The present invention relates to a two-way communication system that adjusts a transmission delay or transmission level when a subscriber station starts its operation over, for example, a two-way CATV network or passive optical star network, and more particularly to a delay or transmission level setting method suitable at a transmission link switching time in a two-way communication system. Moreover, the present invention relates to a delay setting method and a transmission level setting method suitble for two-way communication systems.
In conventional time division multiplex communication systems such as two-way CATV networks, passive optical star networks, or the like, since subscriber stations are respectively located at different distances from a center station, variations occur in the receive timings and the receive signal levels of up-stream signals received by the center station. In order to avoid such a problem, a respective transmission delay is set to each subscriber station by measuring the round-trip propagation delay time between a subscriber station and the center station when the subscriber station starts its operation.
By previously setting respective transmission delays to subscriber stations, the center station can receive up-stream signals transmitted from different subscriber stations at short intervals. In other words, the network transmission efficiency can be improved.
A typical prior-art time division multiplex communication system is disclosed, for example, in JP-A-299278/1992. According to the prior art, a subscriber station first receives a round-trip propagation delay measurement signal transmitted from a center station and then sends back an echo signal to the center station. The center station measures an interval between the time a measurement signal is transmitted and the time an echo signal is received, that is a round-trip propagation delay time.
Variations in the level of an up-stream signal received by a center station restrict the maximum laying distance and the configuration of relay equipment of the network because of the limited receivable signal level of the center station. For that reason, the requirement for receive signal levels is relieved by measuring the receive signal level in the center station when a subscriber station starts to operate and then adjusting it to a constant level.
JP-A-39787/1986, for example, discloses the prior-art level measuring method in which the center station measures the receive level of an echo signal.
In the above-mentioned prior art, where the distance between the center station and a subscriber station is long, a relay station is installed for signal transmission in the middle of the transmission line. In this case, the center station is connected to a relay station via a relay transmission line such as an optical fiber or a coaxial cable. In most cases, the relay transmission line is duplex systematized by a working system and a stand-by system in preparation for a failure of a network. If the relay transmission line is switched to the stand-by system because of a link failure in the relay transmission line in a working system, it is needed to reset the transmission delays or transmission level set amounts of all subscriber stations connected to the relay station.
The number of subscriber stations connected to one relay station depends on the network. However, in the case of the two-way CATV network, the maximum number of subscriber stations is estimated to be about 1000. In order to reset the delay or transmission level, the measurement signal and the measurement response signal must be transmitted and received to respective subscriber stations several times.
Therefore, when a link switching operation occurs in a relay transmission line, resetting respectively the delay or transmission level to each subscriber station very prolongs a failure recovery time of the system.
The present invention is made to solve the abovementioned problems in the prior art. The objective of the present invention is to provide a two-way communication system that can improve the shorten the system recovery time by adjusting the transmission delay or transmission level before an operation of a subscriber station and shortening the system recovery time when a link switching operation between a center station and a relay station occurs, over two-way CATV networks, passive optical star networks and the like.
Another objective of the present invention is to provide a method of setting a delay in a center station that can improve the shorten the system recovery time.
Further objective of the present invention is to provide a method of setting a transmisson level in a center station that can improve the shorten the system recovery time.
In order to achieve the above-mentioned problems, according to the present invention defined in claim 1, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of said subscriber station so as to equalize the round-trip propagation delay measured to said subscriber station to a fixed system delay, and wherein the center station measures a round-trip propagation delay of a subscriber station after a link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure in the relay transmission line, and resets transmission delays of all subscriber stations connected to the relay station at a time based on a difference between the measured delay and a round-trip propagation delay of a signal to the subscriber station before the link switching operation.
According to the two-way communication system of claim 2, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when a link switching operation from the working system to the stand-by system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working station occurs due to a recovery of a failed one of said plural relay transmission lines, the center station measures a round-trip propagation delay of a signal to a subscriber station connected to the relay station involved in the link switching operation and resets round-trip propagation delays of all subscriber stations connected to the relay station involved in the link switching operation at a time, based on the difference between the measured delay and a round-trip propagation delay to the subscriber station before the link switching operation.
According to the present invention defined in claim 3, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of the subscriber station so as to equalize the round-trip propagation delay measured to the subscriber station to a fixed system delay; and wherein the center station measures a round-trip propagation delay of a signal to the relay station when the relay station starts its operation; and wherein when a link switching operation from the working system to the stand-by system occurs due to a failure of the relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, the center station measures a round-trip propagation delay of a signal to the relay station after the link switching operation and then resets transmission delays of all subscriber stations connected to the relay station at a time, based on a difference the measured delay and a round-trip propagation delay to a signal to the relay station before the link switching operation.
According to the two-way communication system of claim 4, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line by a time division multiplex system; and when a link selecting operation from the working system to the standby system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed one of said plural relay transmission lines, the center station measures a round-trip propagation delay of a signal to a relay station involved in the link switching operation and resets transmission delays of all the subscriber stations connected to the relay station involved in the link switching operation, based on the difference between the measured delay and a round-trip propagation delay of a signal to the relay station before the link switching operation.
According to the two-way communication system of claim 5, the transmission delay of the relay station is set so as to equalize the round-trip propagation delay measured to the relay station to a relay station system delay of a fixed value; and when a link switching operation from the working system to the stand-by station occurs due to a failure of the relay transmission line, or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, the center station measures a round-trip propagation delay of a signal to the relay station after the link switching operation and then resets the transmission delay of the relay station so as to equalize transmission delays of all subscriber stations connected to the relay station to those before a link switching operation based on the difference between the measured round-trip propagation delay and a round-trip propagation delay of a signal to the relay station before the link switching operation.
According to the two-way communication system of claim 6, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when a link switching operation from the working system to the stand-by system occurs due to a failure of one of said plural relay transmission lines or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed one of the plural relay transmission lines, the center station measures a round-trip propagation delay of a signal to the relay station after the link switching operation and resets the transmission delay of the relay station involved in the link switching operation so as to equalize transmission delays of all the subscriber stations connected to the relay station involved in the link switching operation to the transmission delay before the link switching operation, based on the difference between the measured round-trip propagation delay and the round-trip propagation delay of a signal to the relay station before the link switching operation.
According to the present invention defined in claim 7, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a receive level when a subscriber station starts its operation and then sets the transmission level of the subscriber station so as to equalize the receive level measured to the subscriber station to a fixed system receive level; and wherein when a link switching operation from the working system to the stand-by system occurs due to a failure of the relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, the center station resets transmission level set values of all subscriber stations connected to the relay station at a time, based on a difference between the measured receive level and a receive level of a signal to a subscriber station after the link switching operation.
According to the two-way communication system of claim 8, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when the link switching operation from the working system to the stand-by system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working station occurs due to a recovery of a failed one of the plural relay transmission lines, the center station measures a receive level of a signal to a subscriber station connected to a relay station involved in the link switching operation and resets transmission levels of all subscriber stations connected to the relay station involved in the link switching operation at a time, based on a difference between the measured receive level and a receive level of a signal to the subscriber station before the link switching operation.
According to the present invention defined in claim 9, a two-way communication system comprises a center station, subscriber stations each for mutually exchanging signals with the center station, a relay station for relaying signals between the center station and the subscriber stations, a relay transmission line connected between the center station and the relay station and duplex systematized by a working system and a stand-by system, and a subscriber transmission line connected between the relay station and the subscriber stations, wherein the center station measures a receive level when a subscriber station starts its operation and then sets the transmission level of the subscriber station so as to equalize the receive level measured to the subscriber station to a fixed system receive level; wherein said center station measures a receive level of a signal to said relay station when said relay station starts its operation; and wherein when a link switching operation from the working system to the stand-by system occurs due to a failure of the relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, the center station measures a receive level of a signal to the relay station after the link switching operation and then resets transmission level set values of all subscriber stations connected to the relay station at a time, based on a difference between the measured receive level and a receive level of a signal to the subscriber station after the link switching operation.
According to the two-way communication system of claim 10, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when a link switching operation from the working system to the stand-by system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working station occurs due to a recovery of a failed one of the plural relay transmission lines, the center station measures a receive level of a signal to a relay station involved in the link switching operation and then resets transmission level set values of all subscriber stations connected to the relay station involved in the link switching, based on a difference between the measured receive level and a receive level of a signal to the relay station before the link switching operation.
According to the two-way communication system of claim 11, the transmission level of the relay station is set so as to equalize the receive level of the signal measured to the relay station to a system receive level of a fixed value; and when a link switching operation from the working system to the stand-by system occurs due to a failure of a relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, the center station measures a receive level of a signal to a relay station after a link switching operation and then resets a transmission level set value of the relay station so as to equalize transmission levels of all subscriber stations connected to the relay station to the transmission level before the link switching operation, based on a difference between the measured receive level and a receive level of a signal to the relay station before the link switching operation.
According to the two-way communication system of claim 12, the center station is connected to plural relay stations via plural relay transmission lines; and all subscriber stations connected to the plural relay stations share a line based on a time division multiplex system; and when a link switching from the working system to the stand-by system occurs due to a failure of one of the plural relay transmission lines or when a link switching operation from the stand-by system to the working station occurs due to a recovery of a failed one of the plural relay transmission lines, the center station measures a receive level of a signal to a relay station after the link switching operation and then resets a transmission level set value of a relay station involved in the link switching operation so as to equalize transmission levels of all subscriber stations connected to the relay station to the transmission level before the link switching operation, based on a difference between the measured receive level and a receive level of a signal to the relay station before the link switching operation.
According to the present invention defined in claim 13, a method of setting a delay in a center station comprises the steps of measuring a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of the subscriber station so as to equalize the round-trip propagation delay measured to the subscriber station to a fixed system delay, measuring a round-trip propagation delay of a subscriber station after a link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure in a relay transmission line, and reseting transmission delays of all subscriber stations connected to said relay station at a time based on a difference between the measured delay and a round-trip propagation delay of a signal to the subscriber station before the link switching operation.
According to the present invention defined in claim 14, a method of setting a delay in a center station comprises the steps of measuring a round-trip propagation delay when a subscriber station starts its operation and then sets a transmission delay of the subscriber station so as to equalize the round-trip propagation delay measured to the subscriber station to a fixed system delay, measuring a round-trip propagation delay of a signal to a relay station when the relay station starts its operation, measuring a round-trip propagation delay of a signal to the relay station after the link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure of a relay transmission line or when a link switching operation from a stand-by system to the working system occurs due to a recovery of a failed relay transmission line, and then reseting transmission delays of all subscriber stations connected to the relay station at a time, based on a difference the measured delay and a round-trip propagation delay to a signal to the relay station before the link switching operation.
According to the present invention defined in claim 15, a method of setting a transmission level in a center station comprises the steps of measuring a receive level when a subscriber station starts its operation, setting the transmission level of the subscriber station so as to equalize the receive level measured to the subscriber station to a fixed system receive level, and resetting transmission level set values of all subscriber stations connected to the relay station at a time, based on a difference between the measured receive level and a receive level of a signal to a subscriber station after the link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure of a relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line.
According to the present invention defined in claim 16, a method of setting a transmission level in a center station comprises the steps of measuring a receive level when a subscriber station starts its operation, setting the transmission level of the subscriber station so as to equalize the receive level measured to the subscriber station to a fixed system receive level, measuring a receive level of a signal to a relay station after the link switching operation when a link switching operation from a working system to a stand-by system occurs due to a failure of a relay transmission line or when a link switching operation from the stand-by system to the working system occurs due to a recovery of a failed relay transmission line, and then resetting transmission level set values of all subscriber stations connected to the relay station at a time, based on a difference between the measured receive level and a receive level of a signal to the subscriber station after the link switching operation.
As to the two-way communication system according to the present invention defined in claim 1, the center station measures a round-trip propagation delay when a subscriber station newly starts its operation and then internally registers a transmission delay set to the subscriber station. When the linking condition is switched from a working system to a stand-by system because of a failure in a relay transmission line connecting the center station to the relay station, or when the linking condition is switched from a stand-by system to a working system because of the recovery of a failed relay transmission line, the center station transmits a round-trip propagation delay measurement signal to an arbitrary subscriber station to newly obtain a round-trip propagation delay.
Successively, the center station calculates the difference between the round-trip propagation delay before a link switching operation and the round-trip propagation delay after a link switching operation and then updates the transmission delay by adding the difference to data on registered transmission delays of all subscriber stations connected to the relay station. After the updating of the transmission delay data, the center station transmits a transmission delay setting signal to all subscriber stations connected to the relay station to update the transmission delay of each subscriber station.
In such a manner, compared with the case where the transmission delay is reset for different subscriber stations, the system recovery time associated with a delay resetting operation can be shortened by detecting the difference between round-trip propagation delay amounts to a subscriber station at a link switching time and then updating the transmission delays of all subscriber stations accommodated by the relay station at a time.
In the two-way communication system according to the present invention defined in claim 2, the center station is connected to plural relay stations via relay transmission lines. When a link switching occurs because of a failure or recovery of one of the relay transmission lines, the center station selects an arbitrary subscriber station connected to the relay station involved in the link switching operation and then transmits a round-trip propagation delay measurement signal to it, thus obtaining a new round-trip propagation delay.
Thereafter, in a similar manner to that defined in claim 1, the difference of a round-trip propagation delay to a subscriber station connected to the relay station in a link switching state is detected and the transmission delays of all subscriber stations accommodated by the relay station are updated at a time. For that reason, the system recovery time associated with a delay resetting operation can be shortened, compared with the case where the round-trip propagation delay is re-measured to the subscriber stations connected all subscriber stations.
In the two-way communication system according to the present invention defined in claim 3, the center station measures the round-trip propagation delay of a signal when a subscriber station newly starts its operation and then internally registers the transmission delay set to the subscriber station.
The relay station has the function of transmitting an echo signal in response to the round-trip propagation delay measurement signal transmitted from the center station. When a link switching operation occurs due to a failure or recovery of the relay transmission line connecting the center station to the relay station, the center station transmits a round-trip propagation delay measurement signal to the relay station to obtain a new round-trip propagation delay.
Next, the center station calculates a difference between a round-trip propagation delay before a link switching operation and a round-trip propagation delay after the link switching operation to the relay station and then adds data on registered transmission delays of all subscriber stations accommodated by the relay station to update them. After the updating of data on the transmission delays, the center station transmits a transmission delay setting signal to each of the subscriber stations connected to the relay station and then updates the transmission delays in the subscriber stations.
In such an operation, compared with the case where a delay resetting operation is performed for different subscriber stations, the system recovery time associated with a delay resetting operation can be shortened by detecting the difference between the round-trip propagation delays from the relay station at a link switching operation and then updating the transmission delays of all subscriber stations accommodated by the relay station at a time.
In the two-way communication system according to the present invention defined in claim 4, the center station is connected to plural relay stations via relay transmission lines. When a link switching operation occurs because of a failure or recovery of one of the relay transmission lines, the center station transmits a round-trip propagation delay measurement signal to the relay station involved in a link switching operation, thus a round-trip propagation delay before a link switching operation and a round-trip propagation delay after the link switching operation to the relay station, updates data on the registered transmission delays of the relay station, and then transmits a transmission delay setting signal to the relay station. The relay station receives the transmission delay setting signal and then updates the internal transmission delay.
In such an operation, the system recovery time associated with a delay resetting operation can be shortened by setting the transmission delay of the subscriber station to a fixed value at a link switching operation and then updating the transmission delay of only the relay station.
In the two-way communication system according to the present invention defined in claim 6, the center station is connected to plural relay stations via relay transmission lines. When a link switching operation occurs because of a failure or recovery of one of the relay transmission lines, the center station transmits a round-trip propagation delay measurement signal to the relay station involved in the link switching operation, thus obtaining a new round-trip propagation delay.
Thereafter, in a similar manner to that defined in claim 5, the difference of a round-trip propagation delay to the obtaining a new round-trip propagation delay.
Thereafter, in a similar manner to that defined in claim 3, the difference of a round-trip propagation delay amount to the relay station involved in the link switching operation is detected and the transmission delays of all subscriber stations accommodated by the relay station are updated at a time. For that reason, the system recovery time associated with a delay resetting operation can be shortened, compared with the case where the round-trip propagation delay is re-measured to the subscriber stations connected all relay stations.
In the two-way communication system according to the present invention defined in claim 5, like the subscriber stations, the relay station has the function of adding a transmission delay. The center station measures the round-trip propagation delay of a signal to the relay station when the relay station newly starts its operation and then internally registers the transmission delay set to the relay station. When a link switching operation occurs because of a failure or recovery of the relay transmission line connecting the center station to the relay station, the center station transmits a round-trip propagation delay measurement signal to the relay station, thus obtaining a new round-trip propagation delay.
Next, the center station calculates a difference between relay station involved in the link switching operation is detected to update the transmission delay. For that reason, the system recovery time associated with a delay resetting operation can be shortened, compared with the case where the round-trip propagation delay is re-measured to all the relay stations.
In the two-way communication system according to the present invention defined in claim 7, the center station measures the receive level of a signal when a subscriber station newly starts its operation and then internally registers the transmission level set to the subscriber station. When a link switching operation occurs from the working system to the stand-by system because of a failure in a relay transmission line connecting the center station to the relay station, or when a link switching operation occurs from the stand-by system to the working system because of the recovery of a failure in the relay transmission line, the center station selects an arbitrary subscriber station and then transmits a level measurement signal to it, thus obtaining a new receive level.
Successively, the center station the difference between the receive level before a link switching operation and the receive level after the link switching operation to the relay station and then updates the previous transmission level by adding the difference to data on registered transmission levels of all subscriber stations connected to the relay station. After the updating of the transmission level data, the center station transmits a transmission level setting signal to all subscriber stations connected to the relay station to update the transmission level of each subscriber station.
In such a manner, compared with the case where the transmission level is reset for different subscriber stations, the system recovery time associated with a transmission level resetting operation can be shortened by detecting the difference between receive levels to a subscriber station at a link switching time and then updating the transmission levels of all subscriber stations accommodated by the relay station at a time.
In the two-way communication system according to the present invention defined in claim 8, the center station is connected to plural relay stations via relay transmission lines. When a link switching operation occurs because of a failure or recovery of one of the relay transmission lines, the center station selects an arbitrary subscriber station connected to the relay station involved in the link switching operation, and transmits a level measurement signal to it, thus obtaining a new receive level.
Thereafter, according to a similar procedure to that defined in claim 7, the difference of a receive level to an arbitrary subscriber station connected to the relay station involved in the link switching operation is detected to update data on the registered transmission levels of all subscriber stations accommodated by the relay station at a time. For that reason, the system recovery time associated with a transmission level resetting operation can be shortened, compared with the case where the receive level is re-measured to subscriber stations connected to all the relay stations.
In the two-way communication system according to the present invention defined in claim 9, the center station measures the receive level of a signal when a subscriber station newly starts its operation and then internally registers the transmission level set to the subscriber station.
The center station also measures the receive level of a signal when the relay station newly starts its operation and then internally holds the receive level of a signal to the relay station.
The relay station has the function of transmitting an echo signal in response to a level measurement signal transmitted from the center station. When a link switching operation occurs because of a failure or recovery of a relay transmission line connecting the center station to the relay station, the center station transmits a level measurement signal to the relay station, thus obtaining a new receive level.
Successively, the center station calculates the difference between the receive level before a link switching operation and the receive level after the link switching operation and then updates the previous transmission level by adding the difference to data on registered transmission levels of all subscriber stations accommodated by the relay station. After the updating of the transmission level data, the center station transmits the transmission level setting signal to subscriber stations connected to the relay station to update the transmission delay of each subscriber station.
In such a manner, compared with the case where the transmission level is reset for different subscriber stations, the system recovery time associated with a delay amount resetting operation can be shortened by detecting the difference between receive levels from the relay station at a link switching time and updating the transmission levels of all subscriber stations accommodated with the relay station at a time.
In the two-way communication system according to the present invention defined in claim 10, the center station is connected to plural relay stations via relay transmission lines. When a link switching operation occurs because of a failure or recovery of one of the relay transmission lines, the center station transmits a level measurement signal to the relay station related to a link switching operation, thus obtaining a new receive level.
Thereafter, according to a similar manner to that defined in claim 9, the difference of a receive level to the relay station involved in the link switching operation is detected to update the transmission levels of all subscriber stations accommodated by the relay station at a time. As a result, the system recovery time associated with a transmission level resetting operation can be shortened, compared with the case where the receive level is re-measured to subscriber stations connected to all the relay stations.
In the two-way communication system according to the present invention defined in claim 11, like each subscriber station, the relay station has the function of adjusting a transmission level. The center station measures the receive level of a signal to the relay station when the relay station newly starts its operation and then internally registers the transmission level set to the relay station. When a link switching operation occurs because of a failure or recovery of a relay transmission line connecting the center station to the relay station, the center station transmits a level measurement signal to the relay station, thus obtaining a new receive level.
Successively, the center station calculates the difference between the receive level before a link switching operation and the receive level after the link switching operation and then updates data on the registered transmission level of the relay station, and then transmits a transmission level setting signal to the relay station. The relay station also receives the transmission level setting signal and then updates the internal transmission level.
In such a manner, the system recovery time associated with a transmission level resetting operation can be shortened by setting the transmission level of each subscriber station at a link switching time to a fixed value and updating the transmission level to only the relay station.
In the two-way communication system according to the present invention defined in claim 12, the center station is connected to plural relay stations via relay transmission lines. When a link switching operation occurs because of a failure or recovery of one of the relay transmission lines, the center station transmits a level measurement signal to the relay station related to a link switching operation, thus obtaining a new receive level.
Thereafter, according to a similar procedure to that defined in claim 11, the difference of a receive level to the relay station related in the link switching operation is detected to update the transmission levels of all subscriber stations. As a result, the system recovery time associated with a transmission level resetting operation can be shortened, compared with the case where the receive level is re-measured to all the relay stations.